Bison are for the most part very companionable creatures. They are group-oriented in the extreme. While you will occasionally encounter a solitary old bull, they are the exception rather than the rule.
When bison roamed the American plains the male-female ratio was much closer to 1:1 than it is now in controlled herds. Nowadays the ratio tends to be 15 or 20:1. Even so, there seems to be quite a bit of family cohesion within a herd, and you will see the same cow/bull combination frequently together.
Calves are ear-tagged as they are born and one rancher noted that consecutively numbered ear tags will go through a sorting chute together for years. These are not siblings, just "group buddies."